The invention relates to an internal combustion engine having at least one cylinder, at the outlet side at least one exhaust line for discharging the exhaust gases via an exhaust-gas discharge system, and at the inlet side at least one intake line for supplying charge air via an intake system, which internal combustion engine is equipped with                at least one exhaust-gas turbocharger comprising a compressor arranged in the intake system and a turbine arranged in the exhaust-gas discharge system,        a bypass line for bypassing the at least one cylinder, which bypass line branches off from the intake system downstream of the compressor so as to form an inlet-side junction and opens into the exhaust-gas discharge system downstream of the turbine, and        at least one exhaust-gas recirculation arrangement which comprises a recirculation line which branches off from the exhaust-gas discharge system upstream of the turbine so as to form an outlet-side junction and which opens into the bypass line.        
Within the context of the present invention, the at least one exhaust line is to be regarded as belonging to the exhaust-gas discharge system, and the at least one intake line is to be regarded as belonging to the intake system.
The invention also relates to a method for operating an internal combustion engine of the stated type which has a liquid-type cooling arrangement, in which a cooling device which is operated with coolant and which is connected to the liquid-type cooling arrangement of the internal combustion engine is provided in the recirculation line of the at least one exhaust-gas recirculation arrangement.
Within the context of the present invention, the expression “internal combustion engine” encompasses diesel engines and spark-ignition engines and also hybrid internal combustion engines which operate with a hybrid combustion process.
In the development of internal combustion engines, it is basically sought to minimize fuel consumption and reduce pollutant emissions. In the prior art, various measures are implemented to achieve said aims. With regard to the pollutant problem, the reduction of nitrogen oxide emissions is of high relevance, in particular in the case of diesel engines. Since the formation of nitrogen oxides requires not only an excess of air but rather also high temperatures, one concept for reducing the nitrogen oxide emissions consists in reducing the combustion temperatures. Here, exhaust-gas recirculation, that is to say the recirculation of exhaust gas from the exhaust-gas discharge system into the intake system, is expedient in achieving this aim, wherein it is possible for the nitrogen oxide emissions to be considerably reduced with increasing exhaust-gas recirculation rate. The exhaust-gas recirculation rate xEGR is determined as follows:xEGR=mEGR/(mEGR+mFresh air)where mEGR denotes the mass of recirculated exhaust gas and mFresh air denotes the fresh air which is supplied, having previously been compressed by means of a compressor, to the at least one cylinder. Within the context of the present invention, therefore, the charge air may also comprise recirculated exhaust gas aside from the fresh air.
To obtain a considerable reduction in nitrogen oxide emissions, high exhaust-gas recirculation rates are required which may be of the order of magnitude of xEGR≈60% to 70%. According to the prior art, to adjust the exhaust-gas quantity to be recirculated (i.e., the recirculation rate), a control element, also referred to as an EGR valve, is provided in the recirculation line. Exhaust-gas recirculation (EGR) may also be utilized for reducing the emissions of unburned hydrocarbons.
The exhaust-gas recirculation arrangement provided is a so-called high-pressure EGR arrangement in which the exhaust gas is extracted from the exhaust-gas discharge system upstream of the turbine and is introduced into the intake system, or into a bypass line which is or can be connected to the intake system, downstream of the compressor.
The internal combustion engine to which the preferred embodiment of present invention relates is equipped not only with at least one exhaust-gas recirculation arrangement but rather also with at least one exhaust-gas turbocharger which comprises a compressor arranged in the intake system and a turbine arranged in the exhaust-gas discharge system. During the operation of an internal combustion engine with exhaust-gas turbocharging and with the simultaneous use of a high-pressure EGR arrangement, there is a basic conflict which consists in that the exhaust gas extracted upstream of the turbine for the purpose of recirculation is by definition not conducted through the turbine, and is thus not available for generating a charge pressure on the inlet side. In the event of an increase in the exhaust-gas recirculation rate, the exhaust-gas flow introduced into the turbine simultaneously decreases. The reduced exhaust-gas mass flow through the turbine leads to a lower turbine pressure ratio, as a result of which the charge pressure ratio also falls, which equates to a smaller compressor mass flow. Aside from the decreasing charge pressure, additional problems may arise in the operation of the compressor with regard to the surge limit of the compressor.
The exhaust-gas quantity conducted through the recirculation line of the EGR arrangement and the exhaust-gas quantity conducted through the turbine are only two examples of fluid flows which, during the operation of an internal combustion engine, must be measured and adjusted by means of control elements.
The turbine of an exhaust-gas turbocharger is often designed as a wastegate turbine with a small turbine cross section, in order to improve the torque characteristic of the internal combustion engine at low rotational speeds. If the exhaust-gas mass flow exceeds a critical value, then, by opening a control element, a part of the exhaust-gas flow is, within the course of the so-called exhaust-gas blow-off, conducted via a bypass line past the turbine or the turbine impeller. The blown-off exhaust gas is another example of a fluid flow which must be adjusted and controlled by means of a control element.
Further examples of fluid flows which must be adjusted, that is to say controlled, arise if the internal combustion engine is equipped with a secondary air injection facility or a liquid-type cooling arrangement in which the cylinder head or cylinder block is provided with coolant-conducting ducts. Since it is not the aim and the purpose of a liquid-type cooling arrangement to extract the greatest possible amount of heat from the internal combustion engine under all operating conditions, it is sought, by means of control elements, to implement control of the coolant flows and thus of the liquid-type cooling arrangement according to demand, which control also makes allowance for the operating modes of the internal combustion engine in which it is more advantageous to extract less heat or as little heat as possible from the internal combustion engine, and if appropriate to introduce heat into the internal combustion engine.
The above statements make it clear that a multiplicity of fluid flows must be adjusted and controlled by means of control elements during the operation of an internal combustion engine. For this reason, concepts are required which arrange the fluid-conducting lines in an advantageous manner and which keep the number of control elements required as low as possible.